a) Field of the Invention
The present invention relates to an electric vehicle (hereafter, called EV ), and especially to an EV provided with a vehicle running motor for every driving wheel, in other words, a wheel motor type EV. More particularly, the present invention relates to a driving controller which is mounted on board the wheel motor type EV in order to control the respective vehicle running motors.
b) Description on the Related Art
"Electric vehicle" is a general term for vehicles driven by electric motors. EVs can be classified according to types of power trains from the vehicle running motors to the respective driving wheels into one-motor type EVs, in which output of one common motor is distributed to right and left driving wheels via a differential gearing and the like, and wheel-motor type EVs, in which an electric motor is arranged to every driving wheel in order to drive the driving wheel corresponding to the electric motor. The wheel-motor type EV is superior to the one-motor type EV in several points. One superior point of the wheel-motor type EV is that such an EV requires only a small sized battery for supplying driving power to the electric motor because a torque transmission/distribution mechanism, such as a differential gearing, is unnecessary and energy consumption is small because of small transmission loss. A further advantage of the wheel-motor type EV is that the respective vehicle running motors can be made small because every electric motor drives only a driving wheel corresponding to that electric motor. A third advantage of the wheel-motor type EV is that integration level is increased (cabin space is enlarged) because the respective vehicle running motors can be built inside the respective driving wheels. The wheel-motor type EVs, in which the respective vehicle running motor are built inside the respective driving wheels, are hereafter called wheel-in-motor type EVs.
A fourth advantage of the wheel-motor type EV is that the output can be controlled separately for every driving wheel. The wheel-motor type EV disclosed in Japanese Patent Laid-Open Publication No. Hei. 5-91607, intend to improve vehicle straight line stability by utilizing this fourth advantage. Specifically, the wheel-motor type EV disclosed in Japanese Patent Laid-Open Publication No. Hei. 5-91607 is provided with a driving controller for separately controlling the rotational speed of the respective vehicle running motors. This driving controller first predicts the motion of the car body utilizing a vehicle motion model prepared beforehand when the expected reference number-of-rotation has actually been supplied to the respective vehicle running motors. In further detail, the yaw-rate of the vehicle about the center of gravity of the vehicle body, that is, the angular velocity of the car body in the yawing direction, is calculated on the basis of the detected values of both of the steering angle and the vehicle speed and according to the vehicle motion model. In Japanese Patent Laid-Open Publication No. Hei. 5-91607, the difference between the predicted yaw-rate derived in this way and the yaw-rate detected by a yaw-rate sensor is called yaw-rate noise. The driving controller disclosed in Japanese Patent Laid-Open Publication No. Hei. 5-91607 supplies the reference number-of-rotation corrected so as to reduce the yaw-rate noise to the respective vehicle running motors.
The yaw-rate noise in Japanese Patent Laid-Open Publication No. Hei. 5-91607 is the value obtained by subtracting unpredictable errors, such as shifts due to differences between dynamically loaded radiuses of right and left tires, difference between loads of right and left wheels, inclination and undulation of roads, wind direction, etc. in the detected values of the steering angle and the car speed from the detected yaw-rate. Therefore, if speed control as disclosed in Japanese Patent Laid-Open Publication No. Hei. 5-91607 can be achieved, straight line stability of a vehicle may be achieved against the respective factors mentioned above in addition to the difference between dynamically loaded radiuses of right and left tires.
However, the control disclosed in Japanese Patent Laid-Open Publication No. Hei. 5-91607 is in fact difficult to realize. First, in order to keep and improve the straight line stability of a vehicle, the predicted yaw-rate of the vehicle motion model should be estimated in real time so as to accurately reflect the behavior of an actual vehicle. On the other hand, EVs are designed so that the rotational speed of the vehicle running motors can be dynamically changed within the wide speed range in order to steadily realize the running speed and the acceleration/deceleration or the like required by a driver, in other words, in order to realize them so that the driver does not feel a sense of unnaturalness. Consequently, the rotational speed of the vehicle running motors and further the vehicle speed may largely or rapidly change in EVs. In such a state, the predicted yaw-rate of the vehicle motion model estimated on the basis of the detected car speed often does not fit the actual car speed. Taking into consideration further that control of electric motors over a wide speed range is difficult in general, it is seen that realization and effects of maintaining and improving the straight line stability of a vehicle according to the art disclosed in Japanese Patent Laid-Open Publication No. Hei. 5-91607 is difficult and limited.
Even if straight line stability of a vehicle can be improved by the control disclosed in Japanese Patent Laid-Open Publication No. Hei. 5-91607, perfect running safety of the vehicle can not be maintained and improved merely by improving the straight line stability of the vehicle. There are other important factors such as factors governing the running stability of a vehicle in the cases of speed changes, turns, and lane changes. In other words, in order to realize perfect running safety, control for maintaining the running stability of a vehicle against slip during speed changing, spin during turn, oversteer/understeer at lane changing is necessary. However, the control disclosed in Japanese Patent Laid-Open Publication No. Hei. 5-91607 can not accomplish perfect vehicle running stability. In other words, the control disclosed in Japanese Patent Laid-Open Publication No. Hei. 5-91607 can not cope with the above-mentioned negative factors such as slip during speed changing, because cornering powers Kf, Kr are presumed as constants, which in actuality, they vary with car speed, acceleration, and angular velocity.